drm/amdgpu: add atomfirmware helper function to query fw cap

Fimware capability was changed from 16 bits to 32 bits
for atomfirmware. add helper funciton to query firmware
capability and cache the value at early stage.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: John Clements <john.clements@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>